1. Field of the Invention
This invention relates to a guide for the magnetic tape of video recording and playback apparatus having a rotary magnetic head, and more particularly, to its grounding structure.
2. Description of the Prior Art
A magnetic video recording and playback apparatus usually has a tape conveying system such as that shown in FIG. 1. A magnetic tape 2 leaving a supply reel 1 travels past a guide post 3A, a tension post 4, loading posts 5A to 5C, a sound erasing head 6 and a guide post 3B, and makes about a half turn spirally around a magnetic tape guide drum 7. The drum 7 is provided with at least one head for shortwave recording and playback on the magnetic tape 2. The tape 2 leaving the drum 7 travels past a guide post 8A, a sound re-recording head 9, a guide post 8B, a capstan 10, a pinch roller 11, a loading post 12 and a guide post 13, and is wound on a takeup reel 14. A half of a cassette housing is shown at 15, and the lower portions of loading grooves are shown at 16 and 17.
The magnetic tape guide drum 7 is constructed as shown by way of example in FIG. 2. The drum 7 comprises a stationary drum portion 18 and a rotary drum portion 19 disposed coaxially with the stationary drum portion 18. The rotary drum portion 19 is equal in outside diameter to the stationary drum portion 18. The stationary and rotary drum portions 18 and 19 have a clearance 20 therebetween. A supporting shaft 22 is secured to the center of the rotary drum portion 19 by a sleeve 21. A cylindrical permanent magnet 25A surrounds the shaft 22 and is supported on its lower end by a bearing sleeve 23 and a housing 24. A pair of bearing members 26A and 26B are provided in the stationary drum portion 18 for supporting the shaft 22 and thereby the rotary drum portion 19 rotatably. The stationary drum portion 18 is also provided with a drive coil 25B which is encircled by the permanent magnet 25A. The magnet 25A and the coil 25B constitute a motor. The rotary drum portion 19 is provided with a magnetic head 27 having a signal writing or reading terminal which is directed radially outwards in the clearance 20. A pair of rotary transformers 28A and 28B are provided between the stationary and rotary drum portions 18 and 19 so that recording or playback signals for the magnetic tape 2 may be transmitted through the transformers 28A and 28B from the stationary drum portion 18 connected to an electrical signal system.
Static electricity is generated by the sliding contact of the magnetic tape 2 with the rotary drum portion 19, and has an adverse effect on a very small signal for recording or playback. The static electricity may be transmitted to the bearing members 26A and 26B and result in irregular sliding motion for the shaft 22. For grounding purposes, a spring member 30 having a sliding contact 29 resting on the shaft 22 is connected to a connecting member 31 which is attached to a chassis not shown, or the like. The end of the shaft 22 on which the sliding contact 29 rests, however, does not have a mirror surface, but generates a sliding noise. It is necessary that the spring member 30 and the connecting member 31 be sufficiently long not to interfere with the operation of the rotary drum portion 19. This requirement makes the whole apparatus undesirably tall and large. The structure of FIG. 2 has also the disadvantage of including the various members for grounding the shaft 22 and the members for securing them, and accordingly can be expensive.